Recent approaches in the field of tissue engineering involve the use of polymeric biomaterials as cell scaffolds, which provide cells with a three-dimensional support material on which to grow. Despite a recent expansion in the design and development of suitable scaffold materials, there is still a lack of suitable scaffold materials with systematically variable properties. Without suitable materials available with a wide range of properties to serve as scaffolds for tissue engineering, it is unlikely that the field will achieve its full potential.
Advances in polymer chemistry and materials science have spawned the development of numerous biomaterials and scaffolding methods that have potential uses in a wide range of tissue engineering applications. Several criteria must be achieved in the design of a biomaterial. First, the material must be biocompatible. That is, it must not promote an immune, allergenic, or inflammatory response in the body. Also, a method must exist to reproducibly process the material into a three-dimensional structure. Adhesive properties of the surface of the biomaterial must permit cell adhesion and promote growth. In addition, the biomaterial should have a high porosity to facilitate cell-polymer interactions, improve transport properties, and provide sufficient space for extracellular matrix generation. Finally, depending upon the particular application, the biomaterial should be biodegradable with an adjustable degradation rate so that the rate of tissue regeneration and the rate of scaffold degradation can be matched.
Natural materials, such as collagen and many polysaccharides, generally exhibit a limited range of physical properties, are difficult to isolate, and cannot be manufactured with a high degree of reproducibility. However, natural materials often are more biocompatible and may even have specific biologic activity. Synthetic materials, on the other hand, can be cheaply and reproducibly processed into a variety of structures and the mechanical strength, hydrophilicity, and degradation rates of synthetic scaffolds are more readily tailored. However, synthetic polymers can cause inflammatory responses when implanted in the host. Recent tissue engineering endeavors have attempted to combine properties of both natural and synthetic polymers in the design of a suitable scaffold.
Polylactide (PLA), polyglycolide (PGA) and their copolymers (PLGA) are polyesters based on naturally occurring lactic and glycolic acids (α-hydroxy acids). They have been used as biodegradable sutures and implantable materials for more than two decades. They are biocompatible and biodegradable, and these polymers have a history of use as polymer scaffolds in tissue engineering. However, their highly crystalline and hydrophobic nature makes it difficult to control their biodegradation process and mechanical properties. Moreover, because of the lack of pendant functional groups, it is extremely difficult to modify the surface chemistry of PLA and PGA. For example, proteins and other molecules that may facilitate cell adhesion and growth cannot be easily attached to the backbone of these polymers because there is no chemical “handle” with which to derivatize these substrates. Attempts to introduce functional groups into these types of polymers include copolymerization of the lactide and glycolide cyclic monomers with more easily derivatizable monomers such as cyclic lysine monomers modified by peptide attachments.
Recently, alternating copolymers of α-hydroxy acids and α-amino acids (polydepsipeptides) have been obtained with functional side groups. Additionally, poly(L-lactides) containing β-alkyl α-malate units have been prepared by ring opening copolymerization of L-lactide with a cyclic diester. Major drawbacks remain with these lactide based copolymers including the difficulty in synthesis of cyclic monomers that are used in the copolymerization with lactide and the generally low reaction yields. Thus, the difficult synthesis and the low reaction yields make the commercialization of the modified polylactide biomaterials improbable and make it nearly impossible to tailor chemical, physical, and degradation properties of the final polymer.
Photopolymerization systems have numerous advantages for matrix production. First, photoinitiation allows facile control over the polymerization process with both spatial and temporal control. For example, a liquid macromer solution can be injected into an area of the body, formed into a particular shape, and photopolymerized on demand using a light source. The final polymer hydrogel maintains the shape of that specific area of the body, allowing intimate control over the final shape of the hydrogel and improved adhesion and integration. In addition, the photocrosslinking chemistry creates covalently crosslinked networks that are dimensionally stable.
Known photopolymerization processes, however, suffer from a number of drawbacks, including: the use of a separate initiator specie that is cytotoxic at relatively low concentrations, the difficulty in polymerizing thick samples because of light attenuation by the initiator, the inhibition of the radical polymerization by oxygen present in the air (which slows the polymerization), and the ability to fabricate gels with a diverse range of properties, especially gels with a high water content while maintaining high mechanical strength. Thus, there exists a need for biocompatible hydrogels which can polymerize in the absence of cytotoxic initiators and which can be tailored to have specific chemical, physical, and degradation properties under physiological conditions.